Machine tools which perform machining such as cutting by moving a tool relative to a workpiece are known in conventional art. In such machine tools, numerical control machine tools are known which specify a path of a tool by coordinates of predetermined axes or the like and perform machining by automatically moving the tool with respect to a workpiece. Such a numerical control machine tool can perform machining on a desired tool path and at a desired velocity by describing machine coordinates or moving velocity of a tool on a machining program.
Patent Literature 1 discloses a curved surface cutting method in which a machining surface having a curved shape is obtained by using a milling tool such as a face milling cutter. This document discloses that the surface of an object to be machined is cut by using a milling tool whose rotation locus of cutting edge is circular and by moving the milling tool in a predetermined tool feeding direction in a state in which the rotation axis of the milling tool is inclined with respect to the normal line of a machining surface.
Patent Literature 2 discloses a milling method which performs machining, by progressively bringing a milling tool close to a finished portion from a bulk material, while always removing the cut material, and a method of forming a track for the milling tool. This document discloses that a milling tool is guided along a continuous track whose cross-sectional shape is a spiral from the external contour of a bulk member to a contour portion of a finished member to change the shape of the member progressively.